Distributed lead functionality testing

ABSTRACT

Techniques for performing lead functionality tests, e.g., lead impedance tests, for implantable electrical leads are described. In some of the described embodiments, an implantable medical device determines whether a patient is in a target activity state, e.g., an activity state in which lead impedance testing will be unobtrusive, such as when a patient is asleep, or capture information of particular interest, such as when the patient is active, in a particular posture, or changing postures. The implantable medical device performs the lead functionality test based on this determination. Additionally, in some embodiments, the implantable medical device may group a plurality of measurements for a single lead functionality test into a plurality of sessions, and perform the measurement sessions interleaved with delivery of therapeutic stimulation.

This application claims the benefit of U.S. provisional application No.60/676,187, filed Apr. 29, 2005, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to implantable medical devices and, moreparticularly, to testing of implantable electrical leads.

BACKGROUND

Implantable medical devices may be used to deliver therapeuticelectrical stimulation to patients to treat a variety of symptoms orconditions, such as chronic pain, tremor, Parkinson's disease, epilepsy,incontinence, or gastroparesis. To treat such symptoms or conditions, animplantable medical device may deliver stimulation via electrical leadsthat include electrodes located proximate to the spinal cord, pelvicnerves, or stomach, or within the brain of a patient. In general,implantable medical devices deliver stimulation in the form ofelectrical pulses. Implantable medical devices coupled toelectrode-carrying leads may additionally or alternatively be used tosense electrical activity within a patient.

An electrical lead may carry multiple electrodes, and each electrode maybe coupled to a respective insulated conductor within the lead. Anelectrode, associated conductor, and tissue proximate to the electrodemay form an “electrical path.” The impedance of the various electricalpaths provided by a lead may vary over the life of an implantablemedical device due to, for example, material degradation or tissuegrowth proximate to the electrode. Further, a lead may develop a shortbetween two or more conductors when insulation fails, or a conductor mayfracture due to bending or other stresses placed on the lead by patientmovement or manipulation.

Changes in lead impedance impair the ability of an implantable medicaldevice to effectively sense electrical activity and/or deliverstimulation. Consequently, it may be desired to identify such changes inorder to take corrective action, such as implantation of a new lead, orselection of different electrodes for sensing or delivery ofstimulation. Traditionally, clinicians have used a programming deviceduring an office visit to manually direct an implantable medical deviceto perform a lead integrity or functionality test. Manual leadfunctionality testing may include manually defining a pluralityelectrode combinations and, for each combination, directing theimplantable medical device to measure the impedance presented byelectrical paths the combination, or another electrical parameter forthe combination, such as the current flowing through the electricalpaths. The impedances, currents, or other electrical parameters of thevarious combinations may allow the clinician to identify changes inimpedance or failures of the electrical path associated with aparticular electrode.

Manual testing may be desired to confirm and maintain device efficacy,but is also very tedious. Because of the large number of possibleelectrode combinations that may be available on the one or more leadscoupled to an implantable medical device, testing can take severalminutes. During this time, therapeutic stimulation is generally notavailable, which can result in discomfort for or danger to the patientbecause symptoms are not suppressed.

Furthermore, significant changes in impedance, or other leadfunctionality issues, may occur between clinic visits, and may occurgradually over time. In some cases, such as where an implantable medicaldevice is used for sensing, or to deliver deep brain stimulation, whichare generally not perceivable by the patient, the patient may not detecta degradation of lead functionality. In such cases, the sensing ortherapy may be inadequate for a significant period of time, e.g., untilthe next regularly-scheduled clinic visit, which may pose risks for thepatient.

Also, a conductor short or fracture may be intermittent, and more likelyto manifest during periods when the patient is changing posture, withina particular posture, or otherwise active. In such cases, a clinicianmay not be able to detect a conductor problem with a manual leadfunctionality test performed during an office visit. The existence ofundiscovered conductor problems may limit the effectiveness of therapyand sensing, as discussed above.

SUMMARY

In general, the invention is directed to techniques for testing thefunctionality of implanted electrical leads. One or more implantedelectrical leads may be coupled to a medical device that senseselectrical activity or delivers electrical stimulation via electrodescarried by the leads. The medical device may automatically perform alead functionality test, e.g., without receiving a command to performthe test from a user or programming device, outside of a clinic setting.In some embodiments, the medical device may advantageously perform suchtests in a manner or at a time that may be less likely to disturb thepatient in which the leads are implanted. Further, in some embodiments,the medical device may advantageously perform such tests in a manner orat a time such that the device is more likely to detect impairments ofthe functionality of leads, and particularly impairments that may beintermittent, such as an intermittent short or fracture of one or moreof the conductors with the lead.

In some embodiments, the medical device determines whether a patient isin a target activity state based on a physiological sensor signal, e.g.,one or more accelerometer signals. In such embodiments, the medicaldevice performs a lead functionality test when the patient is in thetarget activity state. A target activity state may be one in which leadfunctionality testing will be unobtrusive because absent therapy willlikely not be noticed, such as when a patient is asleep. Additionally oralternatively, a target activity state may be one during which a leadimpedance test is more likely to capture information of particularinterest. For example, the medical device may perform a leadfunctionality test when the patient is in a particular posture, orchanging postures or otherwise active, which may allow the medicaldevice to identify intermittent shorts and fractures. The medical devicemay perform the tests whenever it is determined that the patient is inthe target activity state, or periodically based on a schedule thatidentifies how frequently lead functionality tests are to occur.

In some embodiments, the medical device may divide a plurality ofmeasurements for a single lead functionality test into a plurality ofsessions, which may be distributed over time. The medical device mayinterleave measurement sessions with delivery of therapeutic stimulationor sensing. By distributing the total time required for a leadfunctionality test over a plurality of distributed sessions, theconsecutive length of time a patient is without stimulation or sensingmay be reduced. For a patient with chronic pain, the shorter timeperiods without stimulation may be bearable, or even unnoticed.

For example, in some embodiments, a lead functionality test includesmeasuring one or more electrical parameters, such as impedance orcurrent, for each of a plurality of combinations of electrodes. Over thecourse of a plurality of sessions, a medical device may make themeasurements for all of the combinations. Each of the sessions includesmeasurements for one or more electrode combinations. The medical devicemay interleave such measurement sessions with, for example, therapeuticstimulation delivery such as individual electrical stimulation pulses orgroups of stimulation pulses.

A lead functionality measurement test may include measurement of valuesfor one or more electrical parameters, such as impedances or currents,associated with one or more of the electrodes carried by the leads. Themedical device may store measured parameter values for later retrievalby a clinician, provide a message to a patient based on the measuredparameter values, and/or modify a therapy based on the measuredparameter values. The medical device may determine whether to performany or all of these functions based on, for example, comparison ofimpedance magnitude or rate of change to a threshold.

The magnitude and rate of change values maintained by the medical devicemay be averages. In some embodiments, the medical device may maintainmultiple average values calculated over longer and shorter periods oftime for comparison to multiple thresholds. A shorter period averagethat exceeds a threshold, for example, may indicate a more severeproblem that requires immediate attention, such as a lead fracture. Insuch case, the medical device may provide an alarm or message to thepatient, e.g., via the implanted medical device or an externalprogrammer, to cause the patient to visit a clinician.

In one embodiment, the disclosure provides a method comprisingdetermining whether a patient is within a target activity state based ona physiological sensor signal, and automatically performing a leadfunctionality test for at least one electrical stimulation leadimplanted within the patient when the patient is in the target activitystate.

In another embodiment, the invention is directed to a system comprisingat least one electrical stimulation lead implanted within a patient, aphysiological sensor that generates a physiological sensor signal, and aprocessor that determines whether the patient is within a targetactivity state based on the physiological sensor signal, and initiatesperformance of a lead functionality test when the patient is within thetarget activity state.

In another embodiment, the disclosure provides a system comprising meansfor determining whether a patient is within a target activity statebased on a physiological sensor signal, and means for performing a leadfunctionality test for at least one electrical stimulation leadimplanted within the patient when the patient is within the targetactivity state.

In another embodiment, the disclosure provides a method for performing alead functionality test for at least one electrical stimulation leadimplanted within a patient comprising defining multiple combinations ofelectrodes, measuring an electrical parameter for each of thecombinations over a series of measurement sessions, and deliveringtherapeutic stimulation to the patient via the electrical lead betweenconsecutive measurements.

In another embodiment, the disclosure provides a system comprising atleast one electrical stimulation lead implantable within a patient and aprocessor. The processor defines multiple combinations of electrodesthat include electrodes carried by the electrical lead, controlsmeasurement of an electrical parameter for each of the combinations overa series of measurement sessions, and directs delivery of therapeuticstimulation to the patient via the electrical lead between consecutivemeasurement sessions.

In another embodiment, the disclosure provides a system comprising meansdefining multiple combinations of electrodes that include electrodescarried by at least one implantable electrical lead, means for measuringan electrical parameter for each of the combinations of electrodes overa plurality of sessions, and means for delivering therapeuticstimulation to the patient via at least some of the electrodes betweenconsecutive measurement sessions.

Embodiments of the invention may provide one or more advantages. Forexample, the invention may allow the duration of clinician office visitsto be reduced because a clinician can receive results from leadfunctionality tests previously and automatically performed by animplantable medical device. In this manner, a clinician may evaluatelead functionality without needing to perform a test during the visit.The invention may also improve patient comfort by allowing testing to bescheduled over multiple short intervals and/or during a time a patientis likely to be sleeping.

Furthermore, the invention may allow for a more complete or detailedpicture of the state of the lead system by providing long term averagesand trends over time. For example, a gradual increase in measuredimpedance may indicate tissue build-up, while a sharp change couldindicate a conductor short or fracture. The invention may also allow anintermittent lead failure to be detected. For example, lead impedancetesting during the time that a patient is anticipated to be active,changing postures, or within a particular posture may detectintermittent failure that only occurs when patient is within suchactivity states. Additionally, the invention may allow for more rapiddetection of lead problems. Once an implantable medical device detects alead problem, it may proactively adjust delivered therapy and/or notifya patient to schedule a clinical visit, thereby minimizing the time thepatient experiences sub-optimal therapy.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example system thatincludes an implantable medical device that automatic performs leadfunctionality tests.

FIG. 2 is a block diagram further illustrating the implantable medicaldevice of FIG. 1.

FIG. 3 is a flow diagram illustrating an example method forautomatically performing lead functionality tests when a patient iswithin a target activity state.

FIG. 4 is a flow diagram illustrating an example method for performing alead functionality test.

FIG. 5 is a timing diagram illustrating therapeutic stimulationdelivered to a patient by an implantable medical device between sessionsof a lead functionality test according to an example embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an example system 10 forautomatically performing lead functionality testing for one or moreelectrical leads. System 10 includes an implantable medical device (IMD)14 that delivers neurostimulation therapy to patient 12, a patientprogrammer 26, and a clinician programmer 20. As will be described ingreater detail below, IMD 14 may perform lead functionality tests basedon a determination that a patient is within a target activity state, anddivide a lead functionality test into a plurality of temporallydistributed sessions.

IMD 14 delivers neurostimulation therapy to patient 12 via electricalleads 16A and 16B (collectively “leads 16”). Leads 16 may, as shown inFIG. 1, be implanted proximate to the spinal cord 18 of patient 12, andIMD 14 may deliver spinal cord stimulation (SCS) therapy to patient 12in order to, for example, reduce pain experienced by patient 12.However, the invention is not limited to the configuration of leads 16shown in FIG. 1, IMDs that deliver SCS therapy, or IMDs that deliverneurostimulation therapy. For example, one or more leads 16 may extendfrom IMD 14 to the brain (not shown) of patient 12, and IMD 14 maydeliver deep brain stimulation (DBS) therapy to patient 12 to, forexample, treat tremor, Parkinson's disease, epilepsy, or psychologicaldisorders. As further examples, one or more leads 16 may be implantedproximate to the pelvic nerves (not shown) or stomach (not shown), andIMD 14 may deliver stimulation therapy to treat sexual dysfunction,urinary or fecal incontinence or gastroparesis. Leads 16 may includelead extensions, as needed.

Further, the invention is not limited to implementation via an implanteddevice, or a device that delivers stimulation. In some embodiments, anexternal medical device, such as an external trial stimulator,automatically performs lead functionality tests in accordance with theinvention. In other embodiments, an implanted or external medical devicemay detect electrical activity within patient 12 via one or more leads,either as an alternative or in addition to delivering electricalstimulation via the leads.

System 10 also includes a clinician programmer 20. Clinician programmer20 may, as shown in FIG. 1, be a handheld computing device. Clinicianprogrammer 20 includes a display 22, such as a LCD or LED display, todisplay information to a user. Clinician programmer 20 may also includea keypad 24, which may be used by a user to interact with clinicianprogrammer 20. In some embodiments, display 22 may be a touch screendisplay, and a user may interact with clinician programmer 20 viadisplay 22. A user may also interact with clinician programmer 20 usingperipheral pointing devices, such as a stylus or mouse. Keypad 24 maytake the form of an alphanumeric keypad or a reduced set of keysassociated with particular functions.

A clinician (not shown) may use clinician programmer 20 to programneurostimulation therapy for patient 12. The clinician may also useclinician programmer 20 to program IMD 14 to later automatically performlead functionality tests in accordance with the invention, e.g., outsideof a clinic environment, as will be described in greater detail below.

System 10 also includes a patient programmer 26, which may, as shown inFIG. 1, be a handheld computing device. Patient programmer 26 may alsoinclude a display 28 and a keypad 30, to allow patient 12 to interactwith patient programmer 26. In some embodiments, display 26 may be atouch screen display, and patient 12 may interact with patientprogrammer 26 via display 28. Patient 12 may also interact with patientprogrammer 26 using peripheral pointing devices, such as a stylus ormouse.

Patient 12 may use patient programmer 26 to control the delivery ofneurostimulation therapy by IMD 14. For example, patient 12 may be ableto select neurostimulation therapy programs, or modify programparameters such as pulse amplitude, width or rate, within limits set bya clinician. Patient programmer 26 may also provide patient 12 withinformation relating to the functional status of IMD 14. For example,patient programmer 26 may receive signals or information relating to theresults of lead functionality testing from IMD 14, and inform patient 12if leads 16 are functioning properly. In the event that leads 16 are notfunctioning properly, patient programmer 26 may automatically adjust thetherapy delivered to patient 12, and/or indicate a problem and advisepatient 12 to schedule a clinical visit. In some embodiments, IMD 14 mayadditionally or alternatively provide alerts to patient 12 orautomatically adjust the therapy based on the results of leadfunctionality testing.

IMD 14, clinician programmer 20 and patient programmer 26 may, as shownin FIG. 1, communicate via wireless communication. Clinician programmer20 and patient programmer 26 may, for example, communicate via wirelesscommunication with IMD 14 using RF telemetry techniques known in theart. Clinician programmer 20 and patient programmer 26 may communicatewith each other using any of a variety of local wireless communicationtechniques, such as RF communication according to the 802.11 orBluetooth specification sets, infrared communication according to theIRDA specification set, or other standard or proprietary telemetryprotocols. Clinician programmer 20 and patient programmer 26 need notcommunicate wirelessly, however. For example, programmers 20 and 26 maycommunicate via a wired connection, such as via a serial communicationcable, or via exchange of removable media, such as magnetic or opticaldisks, or memory cards or sticks. Further, clinician programmer 20 maycommunicate with one or both of IMD 14 and patient programmer 26 viaremote telemetry techniques known in the art, communicating via a localarea network (LAN), wide area network (WAN), public switched telephonenetwork (PSTN), or cellular telephone network, for example.

FIG. 2 is a block diagram illustrating an example configuration of IMD14. IMD 14 may deliver neurostimulation therapy via electrodes 40A-H oflead 16A and electrodes 40I-P of lead 16B (collectively “electrodes40”). Electrodes 40 may be ring electrodes. The configuration, type andnumber of electrodes 40 illustrated in FIG. 2 are exemplary, and otherembodiments may comprise more or less leads, each lead having more orless electrodes than lead 16A and lead 16B (collectively “leads 16”).Further, in other embodiments, leads 16 may have other shapes, such aspaddle-like shapes with electrodes located on one or more sides of thepaddle, or may include complex, multi-dimensional electrode arraygeometries.

IMD 14 includes a signal generation circuitry 42, a processor 44, amemory 46, telemetry circuitry 52, electrical parameter measurementcircuitry 56 and a physiological sensor 58. Electrodes 40 areelectrically coupled to signal generation circuitry 42 via conductorswithin leads 16. Each of electrodes 40 may be coupled to signalgeneration circuitry 42 via a separate insulated conductor (not shown).Each of electrodes 40, its associated conductor, and proximate tissueform an electrical path.

Signal generation circuitry 42 may deliver electrical signals, e.g.,electrical pulses, via two or more of electrodes 40, e.g., two or moreelectrical paths, one or more of which are return paths. Signalgeneration circuitry 42 may include, for example, one or more outputpulse generators, and switches or the like to couple the pulsegenerators to selected electrodes. Signal generation circuitry 42 maydeliver the signal to patient 12 via selected combinations of electrodes40 under the control of processor 44.

Processor 44 controls signal generation circuitry 42 to delivertherapeutic stimulation to patient 12, e.g., neurostimulation therapy inthe form of electrical pulses. Processor 44 may also control signalgeneration circuitry 42 to deliver non-therapeutic signals for leadfunctionality testing, which may also be in the form of electricalpulses, as will be described in greater detail below. In someembodiments, IMD 14 may additionally include signal detection circuitry(not shown) that detects electrical signals within patient 12 viaelectrodes 40. Electrodes 40 may be electrically coupled to such signaldetection circuitry via conductors within leads 16. As discussed above,the invention is not limited to embodiments in which IMD 14 deliverstherapeutic stimulation to patients, and includes embodiments in whichIMD 14 monitors electrical signals within patient 12 instead of or inaddition to delivery of therapeutic stimulation. However, even inembodiments in which IMD 14 does not deliver therapeutic stimulation,IMD 14 may nonetheless include signal generation circuitry 42 to delivernon-therapeutic signals for lead functionality testing.

Processor 44 may control the delivery of therapeutic stimulationaccording to programs or program parameters selected by a clinicianand/or patient using one of programmers 20, 26. For example, stimulationtherapy programs comprising parameters, such as pulse amplitude, pulsewidth, pulse rate and electrode polarity, may be received from one orboth of the programmers and stored in memory 46. Further, adjustments toparameters or selection of programs may be received from theprogrammers, and the programs stored in memory 46 may be modifiedaccordingly. As illustrated in FIG. 2, IMD 14 may include telemetrycircuitry 52 that facilitates communication, e.g., radio-frequency orinductive communication, between processor 44 and programmers 20, 26.

Processor 44 also automatically initiates lead functionality testingaccording to lead functionality test instructions 48, which are storedin memory 46. Processor 44 also stores lead functionality test results50 in memory 46. Processor 44 may receive lead functionality testinstructions 48 from clinician programmer 20 via telemetry circuit 52during programming by a clinician. Lead functionality test instructions48 may include information identifying combinations of electrodes 40 forlead functionality testing, and instructions that indicate when toperform a lead functionality test.

Processor 44 may include any one or more of a microprocessor, digitalsignal processor (DSP), application specific integrated circuit (ASIC),a field-programmable gate array (FPGA), or equivalent discrete orintegrated logic circuitry. Memory 46 may store program instructionsthat, when executed by processor 44, cause processor 44 and IMD 14 toprovide the functionality attributed to them herein. Memory 46 mayinclude any volatile, non-volatile, magnetic, optical, or electricalmedia, such as any one or more of a random access memory (RAM),read-only memory (ROM), non-volatile RAM (NVRAM),electronically-erasable programmable ROM (EEPROM), flash memory, or thelike.

A lead functionality test may include testing a plurality ofcombinations of electrodes 40. In general, it is desirable to test eachpossible combination of two of electrodes 40 during a lead functionalitytest. However, combinations of more than two of electrodes 40 arepossible. A combination may also include only one of electrodes 40 andan electrode integrated with the outside shell, housing, or “can,” ofIMD 14, but such monopolar testing may not provide evidence of a shortbetween two conductors in leads 16.

For each combination, processor 44 may control signal generationcircuitry 42 to deliver a non-therapeutic, e.g., sub-threshold, pulsevia the electrodes 40 of the combination, and control parametermeasurement circuitry 56 to measure a value of an electrical parameterduring the pulse. A sub-threshold pulse may be, for example, a pulsehaving an amplitude or pulse width significantly lower than that oftherapeutic stimulation pulses. Because of their low amplitude and/orpulse width, such dedicated pulses may not result in any therapeutic oradverse effects, e.g., may not be above a threshold sufficient toactivate any nerves or other tissues, and therefore may be referred toas “sub-threshold” pulses. The measured electrical parameter may be, forexample, the impedance presented by the combination of electrodes or thecurrent through the combination of electrodes during delivery of thepulse. Parameter measurement circuitry 56 may include resistors,capacitors, or other known circuitry for sampling and/or holding a valueof an electrical parameter, which may be coupled in series or parallelwith signal generation circuitry 42 for measurement of one or both ofvoltage or current when the pulse is delivered by the circuitry.

Processor 44 may determine the impedance or current based on themeasured voltage and/or current using any of a variety of knowtechniques. For example, in some embodiments, signal generationcircuitry 42 delivers a voltage pulse with a decay, and measurementcircuitry 56 samples and holds the final voltage value of the pulse atthe end of the pulse. Based on the initial, e.g., programmed, voltagefor the pulse, and the sampled final voltage, processor 44 may determinethe impedance presented by the combination of electrodes using knowntechniques, such as those described in commonly-assigned U.S. Pat. No.6,978,171, which issued to Goetz et al. on Dec. 20, 2005, which isincorporated by reference herein in its entirety. Equations or the likeused by processor 44 to determine the impedance or current may be storedin memory 46.

Alternatively, lead functionality test instructions 48 may instructprocessor 44 to perform a lead functionality test, e.g., impedance test,specifically with the one or more combinations (two or more) ofelectrodes 40 currently being used to deliver therapy to patient 12. Forsuch testing, processor 44 may control measurement circuitry 56 to becoupled to signal generation circuitry 42 and measure the impedance ofthe combination during a therapeutic pulse.

Lead functionality test instructions 48 contain instructions forprocessor 44 to perform lead testing when patient 12 is within a targetactivity state. Target activity states may include, as examples, asleepor active, e.g., the patient is exercising. Target activity states mayalso include a target posture, or when the patient is changing postures.It may be desirable to perform lead functionality testing during a timewhen a patient is sleeping in order to reduce patient discomfort due tothe absence of neurostimulation therapy during testing. It may also bedesirable to perform lead functionality testing during a time when apatient is active, changing postures, or within a particular posture,because intermittent lead failures might only be detectable during suchactivity states. In some embodiments, lead functionality testinstructions 48 may contain instructions for lead functionality testingduring two or more target activity states.

Physiological sensor 58 generates a signal as a function of patientphysiological parameter, such as activity and/or posture. Processor 44may determine whether patient 12 is within the target activity statebased on the signal generated by sensor 58. As examples, sensor 58 maycomprise electrodes or other known sensors for detecting heart rate orrespiration rate, a motion sensor, e.g., piezoelectric motion sensor, orother known sensor that may provide evidence of a patient's activitylevel. In some embodiments, sensor 58 may be a multi-axis accelerometercapable of detecting patient posture and posture changes, as well asgross body movement and footfalls. Further information regarding use ofmulti-axis accelerometers to determine patient posture may be found in acommonly-assigned U.S. Pat. No. 5,593,431, which issued to Todd Sheldonon Jan. 14, 1997, and is incorporated herein by reference in itsentirety.

Although illustrated as including a single sensor 58, IMD 14 may includea plurality of physiological sensors 58, and processor 44 may determinewhether patient 12 is within a target activity state based on thesignals from the plurality of sensors 58. The one or more sensors 58 maybe located within a housing of IMD 14, as suggested by FIG. 2, orcoupled to IMD 14 via leads or wireless communication.

As examples, processor 44 may determine that patient 12 is changingpostures based on changes in signals output by a multi-axisaccelerometer, or within a target posture based on a comparison ofsignals output by the multi-axis accelerometer to templates orthresholds stored in memory 46. Processor 44 may determine whetherpatient 12 is within a target high activity state or sleeping bycomparing, as examples, one or more of activity counts derived from anaccelerometer or piezoelectric crystal signal, a heart rate, a heartrate variability, a respiration rate, or a respiration rate variabilityto threshold values stored in memory 46. Furthermore, IMD 14 may includeany of the sensors, and processor 44 may determine whether patient isasleep, using any of the techniques described in commonly-assigned U.S.patent application Ser. No. 11/081,786 by Heruth et al., filed Mar. 16,2005, the entire content of which is incorporated herein by reference.

During delivery of a non-therapeutic pulse to an electrode combinationfor lead functionality testing, signal generation circuitry 42 may beunable to deliver therapeutic stimulation. In some embodiments, leadfunctionality test instructions 48 instruct processor 44 to divide alead functionality test that includes a plurality of electrodecombinations into multiple sessions. In this manner, IMD 14 may limitthe length of each testing session, such that patient 12 would notnotice the absence of therapy delivered by the IMD. For example, atesting session may be limited to one second or less.

A non-therapeutic pulse for lead functionality testing, e.g., to measureimpedance for a combination of electrodes, can occur in less thanone-tenth of a second. Therefore, multiple sets of electrodes can betested in a testing session even if the testing session is no more thanone second. However, in some embodiments, as shown in FIG. 5, testingsessions may be limited to only a single combination of electrodes andsingle pulse. Dividing a plurality of electrode combinations for leadimpedance testing into multiple sessions may be especially useful when apatient is in a high activity state, because a patient may experiencethe most discomfort with an absence of neurostimulation therapy when thepatient is at a hightly active level.

Lead functionality test instructions 48 may also include a schedule thatinstructs processor 44 to repeat testing at regular intervals. Forexample, lead functionality test instructions 48 may require that a leadfunctionality test on each possible combination of electrodes occur atleast once a day. In embodiments where lead functionality tests aredivided into multiple sessions, IMD 14 may repeat each session at leastonce a day.

The results of lead functionality testing are stored in memory 46 aslead functionality test results 50. In this manner, lead functionalitytest results 50 may contain a history of lead functionality testing,e.g., measured impedance, current or other values, or averages of suchvalues. Lead functionality test results 50 may also include informationidentifying the time and date the results were obtained, as well asother information relating to the conditions under which the leadfunctionality test was performed. For example, the activity levels orpostures assumed patient 12 during a lead functionality test may bestored in memory in association with the results. A history of leadfunctionality testing may allow IMD 14, or a clinician or patient usingclinician programmer 20 and/or patient programmer 26 to detect anintermittent lead failure or tissue formed around one of electrodes 40.

IMD 14 can transmit the results of lead impedance tests to patientprogrammer 26 and/or clinician programmer 20. In some embodiments, aprogrammer, e.g., patient programmer 26, may interpret the results ofthe tests to determine if the impedances are within acceptable values.In other embodiments, IMD 14 may determine if impedances are withinacceptable values, e.g., stored in memory 46, and communicate thisdetermination to patient programmer 26 and/or clinician programmer 20.For example, high impedance may indicate a conductor fracture or tissuegrowth around an electrode, while relatively low impedance can indicatea short between conductors in a lead. In either case, if the result of alead impedance test indicates degraded lead functionality, theprogrammer may provide an indication of degraded lead functionality.

If test results 50 or other signals received from IMD 14 indicate asignificant change in lead functionality, patient programmer 26 mayprovide a message instructing the patient to schedule a clinical visit.Additionally or alternatively, IMD 14 may include circuitry forcommunication with patient 12, e.g., by emitting an audible, vibratory,or perceivable electrical stimulation signal, and processor 44 may alertpatient 12 of a detected lead fault via such a signal when the resultsindicate a significant change in lead functionality. In either case,system 10 may provide patient 12 with a message indicating that aclinical visit is needed to address a significant lead fault, ratherthan waiting for the lead fault to be discovered when IMD 14 isinterrogated by clinician programmer 20 for lead functionality testresults 50 at the next scheduled clinic visit.

Additionally or alternatively, in some embodiments, system 10 mayinclude devices for networked communcation between IMD 14 and/orprogrammer 26 on one hand, and a remote clinic or other monitoringservice on the other. In this manner, test results, lead faultindications, or other lead functionality information may be more quicklyprovided from an IMD to a clinician or the like, who may determine whatcourse of action to follow to address any changes in lead functionality.In some cases, such a networked system may be used by a clinician toreprogram an IMD remotely, in order to address a change in leadfunctionality without requiring the patient to visit a clinic.

Furthermore, IMD 14, patient programmer 26 and/or clinician programmer20 may modify patient therapy to compensate for degraded leadfunctionality. For example, if an electrode conductor within one ofleads 16 has a fracture, electrodes coupled to conductor may be“locked-out,” such that IMD 14 does not deliver stimulation via theelectrodes, and user cannot direct the IMD to deliver stimulation viathe electrodes. Stimulation therapies provided by IMD 14 can be adjustedto utilize combinations of electrodes that do not include electrodeshaving faulty conductors. If an electrode is surrounded by tissuegrowth, stimulation therapy can be adjusted to increase the amplitudefor stimulation programs that use that electrode. In some embodiments,adjustment to patient therapy may be performed automatically by IMD 14,patient programmer 26 and/or clinician programmer 20. In otherembodiments a clinician or patient may manually adjust patient therapyusing one of the programmers.

If test results 50 or other signals received from IMD 14 indicate asignificant change in lead functionality, processor 44 may automaticallycontrol performance of one or more follow-up measurements, either on thespecific electrodes identified as experiencing a functionality change bythe original test, or all electrodes. The follow-up measurement mayoccur at a scheduled time, or when patient 12 is again within the targetactivity state, e.g, within same posture as the original test. Processor44 may use such follow-up measurements to confirm a lead functionalityproblem prior to taking actions such as, for example, notifying a useror modifying therapy. Processor 44 may require three or more consistenttests before taking such actions.

Lead functionality test results 50 may include a large amount of data.In some embodiments, IMD 14 may keep the results of all leadfunctionality tests in memory 46 for an indefinite period. In otherembodiments, test results 50 may be stored in a compressed format withinmemory 46. For example, IMD 14 may clear memory 46 of lead functionalitytest results 50 once IMD 14 has transmitted the content of leadimpedance test results 50 to patient programmer 26 or clinicianprogrammer 20 in order to minimize the amount of memory 46 required byIMD 14 to store lead functionality test results 50. Other memorymanagement techniques are also possible. For example, in someembodiments, IMD 14 may delete lead functionality test results 50 onlyif instructed by a clinician or after transmitting lead impedance testresults 50 to clinician programmer 20.

Some embodiments provide patient 12 and/or a clinician an additionaloption to manage memory 46, including memory used to store leadfunctionality test results 50. For example a clinician may instruct IMD14 to keep lead functionality test results 50 in memory 46 untiltransmitted to clinician programmer 20, or to keep only the most recentor significant results in the event that memory 46 becomes full. Forexample, significant results could include those that show changes inmeasured impedance for a particular set of electrodes. Further,processor 44 may reduce the size of lead functionality test results 50within memory by maintaining one or more averages for measuredelectrical parameters, such as impedances or currents, rather than eachmeasured value.

FIG. 3 is a flow diagram illustrating an example method forautomatically performing lead functionality testing according to theinvention. For example, the described method may be used by IMD 14 inFIGS. 1 and 2 to automatically perform lead functionality testing.First, an IMD stores instructions for lead functionality testing (60).For example, a clinician may use clinician programmer 20 (FIG. 1) tosend lead functionality testing instructions to the IMD. As describedwith respect to IMD 14 in the description of FIG. 2, such instructionscan contain a variety of commands. For example, the stored instructionsmay instruct IMD 14 to perform lead functionality testing when a patientis within a target activity state. In some embodiments, the storedinstructions may require lead functionality testing for two or moretarget activity states. Additionally, the stored instructions may dividea lead functionality test for a plurality of electrode combinations intomultiple sessions. For example, each testing session may be limited toone second or less, which may reduce patient discomfort caused by anabsence of therapy during the lead functionality testing.

IMD 14 monitors for a target activity state (62). The target activitystate may be defined by the instructions stored by the IMD. Differentembodiments of the invention may provide different techniques fordetermining whether the patient is in a target activity state. Forexample, the activity state of a patient may be determined using a heartrate sensor, respiration sensor, motion sensor, or other physiologicalsensor, as discussed above. In some embodiments, a plurality ofphysiological parameters, and associated sensors and techniques, may beused in combination to improve accuracy in determining the activitystate of the patient. As discussed above, examples of target activitystates are sleeping, active, changing postures, or a particular posture.

When IMD 14 determines that the patient is in a target activity state,the IMD performs lead functionality testing according to theinstructions (64). Lead functionality testing, e.g., lead impedance orcurrent testing, may be performed using any known techniques, such asthose described above with reference to FIG. 2. For example, to performlead impedance testing, the IMD may deliver a non-therapeutic pulse viaa combination of two electrodes, measure final voltage or currentamplitude for the pulse, and determine an impedance for the combinationbased on the measured final amplitude. Testing may be repeated for aplurality of electrode combinations and/or for the same combinations ofelectrodes on multiple occasions according to the instructions stored bythe IMD.

After performing the lead functionality test, the IMD stores the resultsof the test in memory (66). The IMD may also determine if the leadfunctionality test results are within limits defined by the storedinstructions (68). The IMD may compare measured impedance or currentvalues for one or more combinations, or one or more averages determinedbased on such values, to one or more threshold values stored in a memoryof the IMD. Further, the IMD may compare a rate of change for an averageimpedance or current value to one or more threshold values stored in amemory of the IMD. In some embodiments, the IMD may maintain multipleaverage values calculated over longer and shorter periods of time in amemory for comparison to multiple thresholds. A shorter period averagethat exceeds a threshold, for example, may indicate a more severeproblem that requires immediate attention, such as a lead fracture.

If one or more of the test results are outside limits defined by theinstructions, thresholds or other information stored in the IMD memory,the IMD may adjust patient therapy, store an alert that the patient or aclinician will receive the next time a programmer communicates with theIMD, cause a patient programmer to immediately alert the patient, ordirectly provide some other audible, vibratory, or stimulation alert tothe patient, e.g., via the IMD (70). For example, if an electrodeconductor has a fracture, the IMD may stop delivering therapies that usethat electrode. If an electrode has been surrounded by fibrous or othertissue growth, which may cause an increase in the measured or averageimpedances associated with that electrode, the IMD may increase thevoltage or current amplitude for therapies that use that electrode.

The IMD may also determine whether a user has requested the stored leadfunctionality test results, e.g., whether a clinician or patient hasrequested the results using a clinician or patient programmer (72). Inresponse to such a request, the IMD will send the results to theprogrammer 20, 26 or another external device (74), where they may bepresented as a trend diagram, histogram, or any other graph. In someembodiments, the IMD will send lead functionality test results toprogrammer or other a device whenever communicating with such a device,e.g., without receiving a specific request for the results. Further,while the method shown in FIG. 3 illustrates the IMD monitoring for arequest to send stored results after storing a result (66), IMD mayreceive requests to export results stored in a memory at any time.

FIG. 4 is a flow diagram illustrating an example method for performing alead functionality test. More particularly, FIG. 4 illustrates anexample method that may be employed by an IMD or other medical device todivide a plurality of measurements for a single lead functionality testinto a plurality of sessions, which may be distributed over time, andinterleaved with delivery of therapeutic stimulation or sensing.

A lead functionality test may include iteratively combining electrodesfrom one or more leads, and testing each combination. A complete leadfunctionality test may include testing all or a substantial majority ofthe possible combinations, e.g., pairs, of electrodes from one or moreleads. According to the example method, the total number of tests for asingle, complete lead functionality test, e.g., the total number ofcombinations, may be distributed over time in a plurality of discretesessions that are interleaved with electrophysiological sensing ortherapeutic stimulation delivery.

In the illustrated example, the IMD delivers therapeutic stimulation toa patient through electrodes carried by at least one lead (80). Inresponse to determining that a lead functionality test is to beperformed (82), e.g., detecting that the patient is within a targetactivity state, the IMD suspends the delivery of therapeutic stimulation(84). The IMD may then measure one of more electrical parameters for afirst combination of the electrodes (86). For example, the a processorof the IMD 14 may control signal generation circuitry 42 to deliver asub-threshold pulse via the first combination of electrodes, and usemeasurement circuitry 56 measure an impedance for the first combination,as described above. The IMD may then resume delivery of therapeuticstimulation (88). If the IMD determines that further combinations ofelectrodes need to be tested for the present lead functionality test(90), the IMD may again suspend therapy (84), and measure an electricalparameter for a next combination of electrodes (86). The IMD maycontinue suspending, measuring and resuming (84-88) so long as furthercombinations of electrodes need to be tested for the present leadfunctionality test. When the present lead functionality test is complete(90), the IMD may continue to deliver therapeutic stimulation (80) untilit is time to automatically perform another lead functionality test(82).

FIG. 5 is a timing diagram showing amplitude of pulses delivered to apatient by an IMD delivering therapy and performing a lead impedancetest according to an embodiment of the invention. More particularly,FIG. 5 illustrates therapeutic stimulation periods 101A-101N(collectively “stimulation periods 101”) in which stimulation isdelivered in the form of electrical pulses, and lead functionalitytesting pulses 103A-103N (collectively “testing pulses 103”). In otherwords, FIG. 5 illustrates a plurality of measurements for a leadfunctionality test divided into a plurality of sessions over time, whichare interleaved with delivery of therapeutic stimulation. The deliveryof pulses illustrated in FIG. 5 may be a result of an IMD performing theexample method of FIG. 4.

The pulses delivered by an IMD during therapeutic stimulation periods101 may be neurostimulation therapy pulses. In general, the IMDcontinuously delivers therapeutic stimulation, except for shortinterruptions required to perform lead functionality testing with thetemporally-distributed testing pulses 203. Lead functionality testingpulses 203 may be non-therapeutic, e.g., may occur at sub-thresholdvoltage or current amplitudes such that the patient can not feel thepulses. The IMD may deliver each of testing pulses 203 via a differentone of a plurality of electrode combinations that are to be testedduring a lead functionality test. For example, the IMD may deliver oneof pulses 203 for every unique pairing of the electrodes coupled to theIMD.

In the illustrated example, each session includes only a single testingpulse 203, i.e., tests only a single combination of electrodes. In otherembodiments, more than one combination of electrodes may be tested bydelivering more than one pulse 203 during each session. In any case,dividing the testing of a plurality of electrode combinations intomultiple sessions may increase patient comfort by preventing noticeabledisruptions to patient therapy during lead functionality testing.

A duration 102 of each testing session may within a range fromapproximately 200 microseconds to approximately five minutes. Forexample, duration 102 may be less than approximately one second.Duration 102 may be approximately equivalent to a single electricalpulse. A time period 104 between adjacent sessions may be within a rangefrom approximately ten seconds to approximately thirty minutes. Forexample, time period 104 may be greater than approximately thirtyseconds, or greater than approximately one minute.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A method for performing a lead functionality test for at least oneelectrical lead implanted within in a patient comprising: definingmultiple combinations of electrodes; measuring an electrical parameterfor each of the combinations over a series of measurement sessions; anddelivering therapeutic stimulation to the patient via the electricallead between consecutive measurement sessions.
 2. The method of claim 1,wherein measuring an electrical parameter comprises measuring animpedance.
 3. The method of claim 1, wherein a duration of each of thesessions is within a range from approximately 200 microseconds toapproximately five minutes.
 4. The method of claim 1, wherein a durationof each of the sessions is less than approximately one second.
 5. Themethod of claim 1, wherein each of the sessions includes a singleelectrical pulse.
 6. The method of claim 1, wherein wherein a timeperiod between adjacent ones of the plurality of sessions is within arange from approximately ten seconds to approximately thirty minutes. 7.The method of claim 1, wherein wherein a time period between adjacentones of the plurality of sessions is greater than approximately thirtyseconds
 8. The method of claim 1, wherein a time period between adjacentones of the plurality of sessions is greater than approximately oneminute.
 9. The method of claim 1, wherein delivering therapeuticstimulation comprises delivering neurostimulation pulses.
 10. A systemcomprising: at least one electrical lead implantable within a patient;and a processor that: defines multiple combinations of electrodes thatinclude electrodes carried by the electrical lead; controls measurementof an electrical parameter for each of the combinations over a series ofmeasurement sessions, and directs delivery of therapeutic stimulation tothe patient via the electrical lead between consecutive measurementsessions.
 11. The system of claim 10, wherein the electrical parametercomprises an impedance.
 12. The system of claim 10, wherein a durationof each of the sessions is within a range from approximately 200microseconds to approximately five minutes.
 13. The system of claim 10,wherein a duration of each of the sessions is less than approximatelyone second.
 14. The system of claim 10, wherein each of the sessionsincludes a single electrical pulse.
 15. The system of claim 10, whereinwherein a time period between adjacent ones of the plurality of sessionsis within a range from approximately ten seconds to approximately thirtyminutes.
 16. The system of claim 10, wherein wherein a time periodbetween adjacent ones of the plurality of sessions is greater thanapproximately thirty seconds.
 17. The system of claim 10, wherein a timeperiod between adjacent ones of the plurality of sessions is greaterthan approximately one minute.
 18. The system of claim 10, furthercomprising an implantable medical device coupled to the lead thatincludes the processor.
 19. The system of claim 10, wherein theprocessor controls delivery of neurostimulation pulses by theimplantable medical device between each of the sessions.
 20. A systemcomprising: means defining multiple combinations of electrodes thatinclude electrodes carried by at least one implantable electrical lead;means for measuring an electrical parameter for each of the combinationsof electrodes over a plurality of sessions; and means for deliveringtherapeutic stimulation to the patient via at least some of theelectrodes between consecutive measurement sessions.
 21. The system ofclaim 20, wherein the means for measuring an electrical parametercomprises means for measuring an impedance.